Lambda trigger on stream events in DynamoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Lambda event record processing?

Given a DynamoDB stream event with the following record snippet, what will the Lambda function log for the NewImage attribute?

{
  "Records": [
    {
      "eventID": "1",
      "eventName": "INSERT",
      "dynamodb": {
        "NewImage": {
          "UserId": {"S": "123"},
          "Score": {"N": "42"}
        }
      }
    }
  ]
}

Assume the Lambda code logs event.Records[0].dynamodb.NewImage.UserId.S and event.Records[0].dynamodb.NewImage.Score.N.

A "UserId: 123, Score: 42"

B "UserId: S, Score: N"

C "UserId: UserId, Score: Score"

D "UserId: null, Score: null"

💡 Hint

Remember DynamoDB stream records store attribute values as objects with type keys like S for string and N for number.

The NewImage attribute contains the new item data with each attribute wrapped in an object indicating its type. Accessing UserId.S gives the string value "123" and Score.N gives the string "42".

🧠 Conceptual

intermediate

Which event types trigger a Lambda from DynamoDB Streams?

Which of the following DynamoDB stream event types can trigger a Lambda function?

A INSERT, MODIFY, REMOVE

B CREATE, UPDATE, DELETE

C ADD, REPLACE, DELETE

D PUT, PATCH, DELETE

💡 Hint

Think about the standard DynamoDB stream event names.

DynamoDB streams emit events named INSERT (new item added), MODIFY (item updated), and REMOVE (item deleted). These trigger Lambda functions.

📝 Syntax

advanced

Identify the syntax error in this Lambda handler for DynamoDB stream

Which option contains a syntax error in this Lambda function snippet that processes DynamoDB stream events?

exports.handler = async (event) => {
  for (const record of event.Records) {
    console.log('Event ID:', record.eventID);
    console.log('Event Name:', record.eventName);
    console.log('New Image:', record.dynamodb.NewImage);
  }
}

A Missing semicolon after console.log statements

B Arrow function missing parentheses around event parameter

C Incorrect use of >= instead of => in arrow function

D No syntax error; code is valid

💡 Hint

Check the arrow function syntax and console.log usage.

The arrow function syntax is correct with one parameter without parentheses. Semicolons are optional in JavaScript. The code is syntactically valid.

❓ optimization

advanced

Best practice to handle batch size in Lambda triggered by DynamoDB Streams

You want to optimize your Lambda function triggered by DynamoDB Streams to process records efficiently without timing out. Which approach is best?

A Set batch size to 1 to process each record individually

B Set a moderate batch size (e.g., 100) and process records asynchronously with error handling

C Set batch size to 1000 and process all records synchronously in one invocation

D Disable batch processing and poll the stream manually

💡 Hint

Consider balancing throughput and Lambda execution time.

Setting a moderate batch size and processing records asynchronously with error handling balances performance and reliability, avoiding timeouts and partial failures.

🔧 Debug

expert

Why does this Lambda triggered by DynamoDB Streams fail to process some records?

Consider this Lambda function snippet triggered by DynamoDB Streams:

exports.handler = async (event) => {
  event.Records.forEach(async (record) => {
    await processRecord(record);
  });
};

Why might some records not be processed before the Lambda finishes?

A Event object is missing Records array

B Lambda times out before processing starts

C Using forEach with async functions does not wait for promises to resolve

D processRecord function is not awaited properly inside forEach

💡 Hint

Think about how async functions behave inside forEach loops.

Array.forEach does not wait for async callbacks to finish, so the Lambda may exit before all processRecord promises resolve. Using a for...of loop with await or Promise.all fixes this.

Practice

1. What is the main purpose of a Lambda trigger on DynamoDB stream events?

easy

A. To backup the DynamoDB table data to S3

B. To manually update the DynamoDB table from the Lambda console

C. To disable the DynamoDB stream temporarily

D. To automatically run code when data in the DynamoDB table changes

Solution

1. Step 1: Understand DynamoDB streams

   DynamoDB streams capture data changes like inserts, updates, and deletes.

2. Step 2: Role of Lambda triggers

   Lambda triggers run code automatically when these stream events occur.

3. Final Answer:

   To automatically run code when data in the DynamoDB table changes -> Option D

4. Quick Check:

   Lambda triggers = auto-run code on data change [OK]

Hint: Lambda triggers run code automatically on data changes [OK]

Common Mistakes:

• Thinking Lambda triggers require manual execution
• Confusing stream enabling with backup
• Assuming Lambda disables streams

2. Which of the following is the correct way to specify a Lambda trigger for a DynamoDB stream in AWS Console?

easy

A. Enable stream on the table, then add Lambda as a trigger on the stream

B. Create Lambda function first, then enable stream on the table

C. Enable stream on the table and Lambda separately without linking

D. Add Lambda trigger before enabling the stream on the table

Solution

1. Step 1: Enable stream on DynamoDB table

   Streams must be enabled first to capture data changes.

2. Step 2: Add Lambda as a trigger on the enabled stream

   Lambda triggers are linked to the stream to react to events.

3. Final Answer:

   Enable stream on the table, then add Lambda as a trigger on the stream -> Option A

4. Quick Check:

   Enable stream first, then add Lambda trigger [OK]

Hint: Enable stream first, then attach Lambda trigger [OK]

Common Mistakes:

• Trying to add Lambda trigger before enabling stream
• Not linking Lambda to the stream
• Enabling stream and Lambda separately without connection

3. Given this Lambda function snippet triggered by a DynamoDB stream event:

def lambda_handler(event, context):
    for record in event['Records']:
        if record['eventName'] == 'INSERT':
            print(record['dynamodb']['NewImage']['id']['S'])

What will this code output when a new item with id '123' is inserted?

medium

A. It prints '123' to the logs

B. It prints the old image of the item

C. It raises a KeyError because 'NewImage' is missing

D. It prints nothing because eventName is 'MODIFY'

Solution

1. Step 1: Check eventName condition

   The code checks if eventName is 'INSERT', which matches new item insertion.

2. Step 2: Access NewImage id value

   It accesses 'NewImage' dictionary and prints the 'id' string value '123'.

3. Final Answer:

   It prints '123' to the logs -> Option A

4. Quick Check:

   INSERT event prints new id '123' [OK]

Hint: INSERT event uses NewImage to get new item data [OK]

Common Mistakes:

• Confusing NewImage with OldImage
• Assuming eventName is MODIFY instead of INSERT
• Not accessing nested dictionary keys correctly

4. You wrote this Lambda code to process DynamoDB stream events but it throws a runtime error:

def lambda_handler(event, context):
    for record in event['Records']:
        print(record['dynamodb']['NewImage']['id']['S'])

What is the likely cause of the error?

medium

A. The 'id' attribute is always an integer, not a string

B. The event may contain DELETE events without NewImage data

C. The event['Records'] key is missing

D. Lambda functions cannot access DynamoDB streams

Solution

1. Step 1: Understand stream event types

   DELETE events do not have 'NewImage', only 'OldImage'.

2. Step 2: Accessing NewImage without check causes error

   Trying to access 'NewImage' on DELETE events causes a KeyError.

3. Final Answer:

   The event may contain DELETE events without NewImage data -> Option B

4. Quick Check:

   DELETE events lack NewImage causing runtime error [OK]

Hint: Check event type before accessing NewImage [OK]

Common Mistakes:

• Assuming all events have NewImage
• Ignoring DELETE event structure
• Believing Lambda can't access streams

5. You want your Lambda triggered by DynamoDB streams to process only UPDATE events where the attribute 'status' changed to 'completed'. Which approach is best?

hard

A. Filter events in DynamoDB stream settings to only send UPDATE events with 'completed' status

B. Use a separate Lambda for each event type and status value

C. In Lambda, check if eventName is 'MODIFY' and compare OldImage and NewImage 'status' values

D. Enable stream with only KEYS_ONLY to reduce data size

Solution

1. Step 1: Understand stream filtering limits

   DynamoDB streams cannot filter by attribute values, only event types.

2. Step 2: Use Lambda code to check eventName and compare status

   Lambda can check if eventName is 'MODIFY' and compare OldImage and NewImage 'status' to detect change to 'completed'.

3. Final Answer:

   In Lambda, check if eventName is 'MODIFY' and compare OldImage and NewImage 'status' values -> Option C

4. Quick Check:

   Filter in Lambda by event and attribute change [OK]

Hint: Filter attribute changes inside Lambda, not in stream settings [OK]

Common Mistakes:

• Trying to filter by attribute in stream settings
• Using KEYS_ONLY which lacks attribute data
• Creating multiple Lambdas unnecessarily